code: def test_start_callback():
      flag = [False]
  
      class MyCallback(Callback):
          def _start(self, dsk):
              flag[0] = True
  
      with MyCallback():
          get_sync({"x": 1}, "x")
  
      assert flag[0] is True
  
file_path: dask/dask/tests/test_callbacks.py
imports: from __future__ import annotations
  from dask.callbacks import Callback
  from dask.local import get_sync
  from dask.threaded import get
  from dask.utils_test import add
label_desc: Use image node_id 1 for calling a global function with example usage: test_start_callback() without return types